PuSH (PubSubHubbub) is a sorta’ “real-time” notification protocol. Rather than poll for changes from a publisher (RSS), you can tell a PuSH hub to poll the publisher and receive a notification when there is a change. The notification is sent by POSTing to a URL on the subscribers machine. This sucks because it means the subscriber needs to run a client at all times to receive notifications.
Perhaps an alternative is to use Twitter as a PuSH message bus. When a PuSH hub detects an update it can send a direct message to the subscriber’s Twitter feed. Subscribers can use Twitter’s user streaming api to fetch and process these direct messages. The advantage is that Twitter queues the messages until you log in to grab them, and it has a decent streaming API for pushing lots of messages quickly to the subscribers. No need to reinvent the wheel when it’s available for free.